This invention relates to a thickness-measuring device. More specifically, this invention relates to a thickness-measuring device based on a principle that the transmissivity of X-rays depends on the thickness of objects being measured.
FIG. 6 is an illustration to show the basic configuration of a thickness-measuring device. As shown in the figure, upper and lower rails 132 and 133 in a pair are disposed horizontally between right and left frame members 31 and 31 in a pair, a gap left between the upper and lower rails 132 and 133. A belt-like object 1, whose thickness is being measured, is running through the gap in the arrow""s direction.
FIG. 7 is an enlarged view of the main portion of the thickness-measuring device. As shown in FIGS. 6 and 7, an X-ray tube unit 10 is set on the upper rail 132 so as to travel back and forth on, and lengthwise of, the rail. The X-ray tube unit 10 comprises an X-ray tube 11 and a case 12, the former built in the latter. The X-ray tube 11 is set downward in the case 12, which has a hole 12h in its bottom to let X-rays through. An X-ray detector 40 is set on the lower rail 133 so as to travel back and forth on, and lengthwise of, the rail. The X-ray detector 40 comprises a sensor 42 and a light receiver 41, the former set on the latter.
The X-ray tube unit 10 and the X-ray detector 40 travel in synchronism with each other on the upper and lower rails 132 and 133, respectively; accordingly, X-rays from the X-ray tube 11 penetrate the belt-like object 1 and reach the light receiver 41 wherever the X-ray tube unit 10 and the X-ray detector 40 are traveling. The energy of light detected by the sensor 42 of the X-ray detector 40 is transformed into an electric pulse and output through a transmitter xe2x80x9cKxe2x80x9d to a monitor xe2x80x9cDxe2x80x9d. Because the transmissivity of X-rays depends on the thickness of the belt-like object 1, a weak electric pulse is displayed on the monitor xe2x80x9cDxe2x80x9d if the transmissivity is low and a strong electric pulse is displayed if the transmissivity is high. Zones 1A, where paint containing a magnetic material or the like is applied, are thick and zones 1B, where paint is not applied, are thin; therefore, paint-applied zones 1A and non-paint-applied zones 1B can be distinguished from each other.
The above conventional thickness-measuring device has the following shortcomings.
(1) If the width of paint-applied zones 1A and the width of non-paint-applied zones 1B are larger than the width of the sensor 42 of the X-ray detector 40, the electric pulse output from the sensor 42 is sensitive, distinguishing the paint-applied zones 1A from the non-paint-applied zones 1B. If the width of paint-applied zones 1A and the width of non-paint-applied zones 1B are smaller than the width of the sensor 42 of the X-ray detector 40, the electric pulse output from the sensor 42 is insensitive because the light receiver 41 receives simultaneously X-rays coming through both a paint-applied zone 1A and a non-paint-applied zone 1B; therefore, the measuring precision of the thickness-measuring device is low.
(2) Timing belts are used to drive the X-ray tube unit 10 and the X-ray detector 40 on the upper and lower rails 132 and 133, respectively, in the above conventional thickness-measuring device, which poses the problems of poor feeding precision and positional slippage of measuring points due to the aged deterioration of the timing belts and the problem of pollution of measuring environment due to dust from the timing belts.
In accordance with the above, the object of the present invention is to provide a thickness-measuring device capable of measuring the thickness of objects sensitively, precisely if their thickness varies in a narrow range.
According to the first feature of the present invention, there is provided a thickness-measuring device comprising (i) an X-ray tube unit including an X-ray tube, (ii) an X-ray detector unit including an X-ray detector having a sensor, and (iii) a means for driving the X-ray tube unit and the X-ray detector unit back and forth, in synchronism with each other, widthwise of an object whose thickness is measured and which runs through a pathline between the X-ray tube unit and the X-ray detector unit. A mask is set above the sensor of the X-ray detector to cover it partly.
According to the second feature of the present invention, there is provided the thickness-measuring device of the first feature, wherein a plurality of pairs of said X-ray tube units and said X-ray detector units are disposed widthwise of the object and a mask is set above the sensor of each X-ray detector to cover it partly.
According to the third feature of the present invention, there is provided the thickness-measuring device of the first feature, wherein the mask covering the sensor is shaped and disposed so as to expose part of the sensor, whose width is smaller than the whole width of the sensor, to X-rays.
According to the fourth feature of the present invention, there is provided the thickness-measuring device of the first feature, wherein the means for driving the X-ray tube unit and the X-ray detector unit in a pair comprises (i) ball-screw rods in a pair which are disposed parallel to each other and on which the X-ray tube unit and the X-ray detector unit travel, respectively, (ii) linear guides in a pair which are disposed along the ball-screw rods, respectively, (iii) servomotors in a pair which rotate the ball-screw rods, respectively, and (iv) a controller which synchronizes the travel of the X-ray tube unit and the travel of the X-ray detector unit.
The advantage offered by the first feature of this invention is as follows. Because the mask covers the sensor partly, the light-receiving area of the light receiver of the X-ray detector is smaller than the whole area of the sensor. Thus, the measuring area per unit time is reduced. Accordingly, if the thickness of the object varies in a narrow range, the thickness-measuring device measures the thickness sensitively.
The advantage offered by the second feature of this invention is as follows. Because X-ray tube units and X-ray detector units in a plurality of pairs are disposed widthwise of the object, the thickness-measuring device measures the thickness of the object quickly, densely.
The advantage offered by the third feature of this invention is as follows. Because the mask covering the sensor is shaped and disposed so as to expose part of the sensor, whose width is smaller than the whole width of the sensor, to X-rays, the width of the light-receiving part of the light-receiver under the sensor is smaller than the whole width of the sensor. Accordingly, although the sensitivity of the sensor is reduced, there is no practical problem in particular so long as the reduction is confined within a certain range on the one hand, and the measuring area per unit time is reduced effectively on the other hand. Thus, the thickness-measuring device measures the thickness of the object with a high precision, detecting minutes changes of the thickness of the object.
The advantage offered by the fourth feature of this invention is as follows. Because the controller and the servomotors drive the X-ray tube unit and the X-ray detector unit in synchronism with each other on the ball-screw rods, along the linear guides, the thickness-measuring device is free from the problem of poor feeding precision due to the aged deterioration of belts, if used, and hence free from positional slippage of measuring points. Thus, the thickness-measuring device measures the thickness of the object with a high precision. Besides, as the thickness-measuring device does not use a belt, it is free from pollution of measuring environment due to dust from belts.